Electronic leakage resistance detector for an ac control circuit

ABSTRACT

An electronic leakage detector electrically connected across an AC control circuit component and responsive to a preselected range of AC voltages across the component to indicate the presence of leakage resistance paths in the control circuitry. The detector comprises a converter which monitors the AC voltage across the component to provide a DC signal which varies in accordance with the magnitude of the AC input and an electronic logic circuit responsive to a range of DC signals to indicate when the AC voltage is within the preselected range. The leakage detector is physically compact and in no way impairs the conventional operation of the control circuitry to which it is connected.

waited States Patent 1 1 Schnur et a1.

ELECTRONIC LEAKAGE RESISTANCE DETECTOR FOR AN AC CONTROL CIRCUITAssignee:

Filed:

lnventors: Earl J. Schnur, Lake Orion; David Westinghouse ElectricCorporation,

Pittsburgh, Pa.

Feb. 11, 1971 Appl. No.: 114,503

11.8. C1 324/51, 324/133, 340/248 C Int. Cl G0lr 31/02 Field of Search324/51, 133, 72.5,

References Cited UNITED STATES PATENTS llq [ 1 Oct. 30, 1973 PrimaryExaminer-Gerard R. Strecker Attorney-A. T.- Stratton and Clement L.Mcl-lale [57] ABSTRACT An electronic leakage detector electricallyconnected across an AC control circuit component and responsive to apreselected range of AC voltages across the component to indicate thepresence of leakage resistance paths in the control circuitry. Thedetector comprises a converter which monitors the AC voltage across thecomponent to provide a DC signal which varies in accordance with themagnitude of the AC input and an electronic logic circuit responsive toa range of DC signals to indicate when the AC voltage is within thepreselected range. The leakage detector is physically compact and in noway impairs the conventional operation of the control circuitry to whichit is connected.

2 Claims, 1 Drawing Figure ELECTRONIC LEAKAGE RESISTANCE DETECTOR FOR ANAC CONTROL CIRCUIT This invention relates to leakage detectors for ACcontrol circuits and more specifically to an electronic circuit fordetecting the presence of spurious AC voltages across AC controlcomponents which are caused by formation of leakage resistance pathssomewhere in the control circuitry. The improved leakage detector ofthis invention indicates the formation of these leakage paths in theirincipiency so thatthey may be eliminated before they become troublesome.p

Generally such leakage paths are caused by a gradual accumulation ofelectrically conductive contaminants on various electrical componentssuch as are used in machine tool circuitry. During the early stages ofcontamination the problem is not readily apparent because the leakageresistance is very large. However, contin-' ued accumulation reduces theleakage resistance and the problem becomes more critical. As the leakageresistance decreases, partial short circuits occur which can cause acontrol component to operate spuriously.

tronic leakage detector of the present invention connected to a portionof the AC control circuitry ofa machine tool.

Referring now to the drawing, a machine tool comprises a base 12 alongwhich a workpiece 14 is being horizontally moved toward a cutter 16 bymeans of a ram 18. Ram 18 is driven by a power cylinder 20 which ishydraulically connected to a valve 22 which is in turn actuated by theapplication of voltage to a solenoid 24. Solenoid 24 is electricallyconnected in series circuit with an AC power supply 26 and a normally aopen limit switch 28 which is mounted on base 12. The

For example, an accumulation of contaminant between v a terminal of alimit switch and the grounded limit switch cover can form a leakageresistance path to ground thereby partially shorting out the switch.Thus,

even when the switch is open the devicewhich it controls is susceptibleto spurious actuation. Normally a control component does not responduntil the voltage applied to it reaches an appreciable level. However,to prevent a control malfunction because of leakage resistance, it isnecessary to detect spurious voltages before they reach this appreciablelevel.

ln the past, leakage paths have been usually detected by directlymeasuring leakage resistance. While such an approach can besatisfactory, it is at best an indirect method of indicating the actualproblem, namely the presence of spurious AC voltage across theelectrical component. A more direct method for solving this problem isto continuously monitor the voltage across the control component toindicate when the leakage reaches a dangerous level. However, becausefull line voltage is applied to the control component when it is beingactuated, the apparatus for monitoring the voltage across the componentmust discriminate against this condition.

Accordingly an important object of this invention-is the provision of animproved leakage detector for AC control circuitry which monitors thevoltage across an electrical circuit component and which is responsiveto a limited range of voltages to indicate the presence of leakageresistance in the control circuitry.

It is another'object of the invention to provide an improved electronicleakage detector for AC circuitry which responds to varying levels of ACleakage voltage to provide varying levels of DC output voltage which arelogically processed by electronic logic elements to indicate when theleakage voltage lies within a predetermined leakage range to therebyindicate leakage re sistance in the control circuitry.

Likewise, it is an object of the invention to provide an improvedelectronic leakage detector for AC circuitry which is inexpensive tomanufacture, is highly reliable and occupies a small amount of space.

Other objects and features of the invention will be apparent in thespecification and the drawing which shows the electrical schematicdiagram of the elecactuating lever 30 of limit switch 28 is positionedto be tripped by a cam 32 on ram 18 such that switch 28 is closed whenthe ram is at one limit of travel. Closure of switch 28 causes solenoid24 to be energized by power supply 26. As a result, valve 22 is shiftedto thereby arrest the movement of ram 18.

The leakage detector 33 of the present invention is connected by inputleads 36 and 38 directly across solenoid 24. Leakage detector 33comprises an AC-DC converter 34 which is in turn connected to a logicsection 67. Logic section 67 comprises in part a high level detector 68and a low level detector 70 which are connected in parallel fashion tothe output of converter 34 by lines 65 and 65'. In turn the output ofdetector 68 is supplied through a logic inverter 72 to one inputterminal of an and-gate 74. The output of detector 70 is supplieddirectly to the other input terminal of gate 74. The output 75 of gate74 indicates when leakage in the AC circuit has reached a predeterminedlevel.

The input circuit of converter 34 comprises a resistor 44 and theprimary coil 36 of a transformer 48. The secondary of transformer 48,whose center tap is connected to DC common 51, is connected through twodiodes 52 and 54 whose cathodes are tied together at one terminal of aresistor 56. The other terminal of resistor 56 is connected to the baseof a transistor 58. The emitter of transistor 58 is connected to DCcommon 51 and its collector is supplied from a DC power supply 60through a resistor 62. In addition, the base of transistor 58 isconnected to its collector and to its emitter by a capacitor 64 and aresistor 66, respectively. The output of converter 34 is taken acrossthe collector and emitter of transistor 58 and is supplied along lines65 and 65' to detectors 68 and 70 as previously indicated.

The operation of leakage detector 33 is now described for each of threeoperating conditions; namely, (1) when limit switch 28 is open and thereis no leakage in the control circuitry, (2) when switch 28 is closed soas to apply full line voltage across solenoid 24, and (3) when limitswitch 28 is open and there is leakage in the control circuitry withinthe range to which the leakage detectoris responsive. The operation ofconverter 34 is first set forth and the response of logic section 67 tothe operation of converter 34 will be subsequently considered.

In the first condition, since there is no voltage input to converter 34,the output voltage thereof is determined by the circuitry connected totransistor 58 comprising resistors 62 and 66, capacitor 64 and powersupply 60. The values of these components are such that capacitor 64becomes charged to the same voltage as that of supply 60 and in thisstate operates to maintain transistor 58 non-conducting. With transistor58 non-conducting, the output voltage of converter 34 is essentiallyequal to the voltage of supply 60. This condition remains stable untilthe second condition occurs, that is, the closure of switch 28.

With the AC voltage of source 26 applied to the input circuitry ofconverter 34, a series of single polarity pulses is applied throughresistor 56 to the base of transistor 58. The magnitude of the pulses iscontrolled by resistor 44. The single polarity feature of these pulsesis obtained by using the rectifying connection of diodes 52 and 54 totransformer secondary 50. For example, if 110 VAC 60 CPS is supplied toconverter 34 then pulses of alternating polarity are supplied to primary46 at a rate of 120 pulses per second. Increasing the resistance ofresistor 44 causes the alternating pulses to decrease in amplitude andvice versa, but does not change the pulse repetition rate.

The appearanceof the pulses at the base of transistor 58 biases thetransistor for conduction. As a result, collector current issimultaneously drawn from capacitor 64 directly and from source 60through resistor 62. Because a controlled impedance path is now presentthrough the collector-emitter circuit of transistor 58, capacitor 64which in part controls the fall rate of the collector voltage dischargesat a predetermined rate. Consequently, the output of converter 34 fallsto a low voltage level. While the current available from the first pulsemay be sufficient to completely discharge capacitor 64, it should beappreciated that several pulses may be required depending upon thevalues of resistors 56 and 66 in relation to the amplitude of therectified voltage pulse at the cathodes of diodes 52 and 54. As long aspulses from the input circuitry of converter 34 keep appearing at thebase of transistor 58, the output voltage of converter 34 is maintainedat its low level. When the pulses disappear, transistor 58 still remainstempoily maintain transistor 58 in the conducting state. How- I ever,when capacitor 64 once again becomes fully charged, transistor'5 8 isrendered nonconducting and the output of converter 34 returns to thehigh voltage level.

Turning now to the third situation, because of leakage in the ACcircuit, the voltage across solenoid 24 lies somewhere in the range towhich leakage detector 33 is constructed to respond to indicate aleakage condition. For example, such a leakage condition can occur whencontaminants have accumulated between contact 28a of limit switch 28 andthe grounded limit switch cover 28b. Voltages which are in the range ofapproximately 20 percent to approximately 70 percent of the full linevoltage comprise a suitable range. The 20 percent figure is sufficientlyhigh to indicate an incipient leakage problem, yet sufficiently low toprevent spurious operation of the circuit component. The 70 percentfigure is sufficiently high to provide a wide leakage response'range,yet sufficiently low so that variations in full line voltage do notprovide a false indication of a non-leakage condition. In this fashionthe leakage detector discriminates against the normal operatingcondition when full line voltage is applied to the circuit component.

Generally converter 34 provides a DC output voltage whose level variesinversely in accordance with the level of voltage across solenoid 24.More specifically, when no AC voltage is being applied to converter 34,transistor 58 is non-conducting and the output of converter 34 ismaintained at a high voltage level substantially identical to thevoltage of power supply 60. As the amplitude of the AC input toconverter 34 increases, the voltage output of converter 34 decreases.When the full line voltage is applied to converter 34, the output is ata low, almost zero, voltage. Thus, when the voltage across solenoid 24is equal to approximately 20 percent of full line voltage, the output ofconverter 34 is at a first signal voltage level between the high and lowvoltage levels. When the voltage across solenoid 24 is equal toapproximately percent of full line voltage, the output of converter 34is at a second signal voltage level significantly lower than the firstsignal voltage level but still between the aforementioned high and lowvoltage levels.

Describing now the operation of logic section 67 in response to theoutput of converter 34, level detector 70 is a conventional logicelement which is repsonsive, that is provides a logical output signal,whenever the output voltage of converter 34 is less than the firstsignal voltage level. Hence, the presence of a logical output signal atthe output of detector 70 indicates that at least 20 percent of fullline voltage is appearing across solenoid 24. Level detector 68 providesa logical output whenever the output voltage of converter 34 is lessthan the second signal voltage level. However, the output of detector 68is logically inverted by inverter 72 and consequently inverter 72provides a logical output signal whenever the voltageacross solenoid 24is less than 70 percent of full line -voltage. Because and-gate 74 isresponsive only to the concurrence of signals from detector 70 andinverter 72, a leakage indication output therefrom occurs only whenthevoltage across solenoid 24 is between 20 percent and 70 percent of fullline voltage.

Therefore, leakage detector 33 successfully discriminates against theproper operation of AC control circuitry to respond only when there is aspurious voltage across solenoid 24 of from 20 percent to 70 percent offull line voltage to thereby indicate a leakage condition. It should bepointed out that whereas during normal switching of solenoid 24 theapplied voltage passes through this 20 percent to 70 percent range, theduration of voltage within this range is so short that detector 33cannot respond to it. It should be appreciated that this range may bevaried accordingly by varying the parameters of the detector circuitry.Similarly, the disclosed construction of detector 33 may be modified invarious fashions to provide identical operation. For example, converter34 could be constructed to provide an increasing output voltage inresponse to an increasing AC input voltage with corresponding changesbeing made in logic section 67. Moreover, the shape of the pulsesapplied to transistor 58 may be varied by substitution of differentcomponents in the input circuitry of converter 34, and the values ofresistors 62 and 66 and capacitor 64 may be modified to respond to thenew pulse characteristics so that basic operation of converter 34 isunchanged. It should also be appreciated that the logic elements oflogic section 67 are conventional and may be readily constructed in anysuitable manner as would be known to one skilled in the art.

By way of example, the following components have been successfullyemployed in converter 34:

resistor 44 33 kohms diodes 52, 54 1N914B resistor 56 47 ohms transistor58 2N3643 power supply 60 +24 VDC resistor 62 2.7 kohms capacitor 64 1micro fd. resistor 66 680 ohms transfonner 48 Better Coil andTransformer Corp. Model No. 8317023 The use of these specific componentsis intended to be merely exemplary to set forth a specific functionalembodiment which has been successfully operated.

We claim:

1. In a leakage detector for AC control circuitry of the type comprisingan electrical component selectively operable by connection to a powersource of predetermined AC voltage, the combination comprising inputcircuit means connected across said component for receiving an AC inputvoltage, converter means electrically connected to said input circuitmeans for providing a DC output signal whose voltage level varies inaccordance with the level of the AC input voltage and electronic logicmeans electrically connected to said converter means and beingresponsive to a preselected range of the DC output signal whenever themagnitude thereof is greater than zero and less than the predeterminedvoltage value of said power source for providing a leakage signalwhenever the DC output signal is within said preselected range for apredetermined minimum period of time, said preselected range beingdefined by first and second DC output signal levels corresponding tofirst and second levels of the AC input voltage respectively, saidelectronic logic means comprising first and second detector meansresponsive to said first and second output signal levels respectively,means for logically inverting the output of one of said detector meansand means responsive to the coincidence of the output of said invertermeans and the output of the other of said detector means for indicatingwhen-the AC input signal is between said first and second levels for apredetermined minimum period of time, whereby said leakage signal isprovided, said electronic logic means including electrical circuitelements which are DC logic elements only, said electronic logic meansresponding to said DC output signal in said range if said DC outputsignal resides in said range continuously for a period of time greaterthan said predetermined minimum value to provide said leakage signal,and not responding to said DC output signal in said range if said DCoutput signal resides in said range continuously for a period of timeless than said predetermined rninimum value, during which lesser timeperiod voltages resulting from normal operation of the control circuitrymay occur.

2. The detector called for in claim 1 wherein said first and secondlevels of the AC input voltage are approximately 20 percent and percentrespectively of the predetermined voltage of the power source.

1. In a leakage detector for AC control circuitry of the type comprisingan electrical component selectively operable by connection to a powersource of predetermined AC voltage, the combination comprising inputcircuit means connected across said component for receiving an AC inputvoltage, converter means electrically connected to said input circuitmeans for providing a DC output signal whose voltage level varies inaccordance with the level of the AC input voltage and electronic logicmeans electrically connected to said converter means and beingresponsive to a preselected range of the DC output signal whenever themagnitude thereof is greater than zero and less than the predeterminedvoltage value of said power source for providing a leakage signalwhenever the DC output signal is within said preselected range for apredetermined minimum period of time, said preselected range beingdefined by first and second DC output signal levels corresponding tofirst and second levels of the AC input voltage respectively, saidelectronic logic means comprising first and second detector meansresponsive to said first and second output signal levels respectively,means for logically inverting the output of one of said detector meansand means responsive to the coincidence of the output of said invertermeans and the output of the other of said detector means for indicatingwhen the AC input signal is between said first and second levels for apredetermined minimum period of time, whereby said leakage signal isprovided, said electronic logic means including electrical circuitelements which are DC logic elements only, said electronic logic meansresponding to said DC output signal in said range if said DC outputsignal resides in said range continuously for a period of time greaterthan said predetermined minimum value to provide said leakage signal,and not responding to said DC output signal in said range if said DCoutput signal resides in said range continuously for a period of timeless than said predetermined minimum value, during which lesser timeperiod voltages resulting from normal operation of the control circuitrymay occur.
 2. The detector called for in claim 1 wherein said first andsecond levels of the AC input voltage are approximately 20 percent and70 percent respectively of the predetermined voltage of the powersource.